The goal of this project is to provide support and continued mentorship as I develop into an independent physician-scientist studying pulmonary fibrosis and the role of epithelial- mesenchymal interaction in the pathogenesis of the disease. The curriculum is specifically designed to strengthen my theoretical and practical knowledge base in murine models of lung fibrosis, genetic and phenotypic analysis, and human lung cell biology. This will be achieved through hands on laboratory experience, didactic training, presentation of data and results, interactions with experts in related fields, and mentorship from a dedicated advisory committee. As outlined in this proposal, I will have at least 75% of time protected for research. During this time, I will be able to gain practical expertise in a number of new techniques, including flow cytometry, fluorescence activated cell sorting (FACS), assessment of murine respiratory mechanics, and human lung cell isolation and culture. I will also build my knowledge base in relevant areas of genomics and gene expression analysis. I am well positioned to successfully complete the work outlined in the proposal given my experienced mentors and the rich and dynamic research environment in which I will perform the work. Research Project: Idiopathic pulmonary fibrosis (IPF) is a disease characterized by severe and progressive alveolar fibrosis that occurs most often in older individuals and that has no effective treatment short of lung transplant. Based upon human genetic studies revealing a link between mutations in genes specific to type 2 alveolar epithelial cells (AEC2s) and familial forms of pulmonary fibrosis, it is accepted that abnormalities in the alveolar epithelium are associated with disease development. We lack knowledge, however, of how these epithelial abnormalities trigger the exuberant response from the underlying mesenchyme that leads to significant and progressive scar formation. We also are unaware of which cells give rise to the pathologic myofibroblasts that are characteristic of the disease. I have identified a unique population of fibroblasts in the lung mesenchyme that are likely to be a source of myofibroblasts. These cells, which express PDGF receptor alpha (Pdgfra), lie in close proximity to AEC2s and, after bleomycin lung injury, proliferate, up-regulate myofibroblast markers, and express pro-fibrotic mediators such as hyaluronic acid synthase 2 (Has2). I have also shown that these cells support the clonal growth and differentiation of murine AEC2s in a 3-dimensional murine culture system, suggesting that they are critical components of the alveolar epithelial cell niche. I hypothesize that Pdgfra+ fibroblasts are a source of myofibroblasts following bleomycin lung injury, that these cells produce pro-fibrotic mediators such as hyaluronan (HA) and Has2 when triggered, and that these cells are critical components of the human AEC2 niche during homeostasis. I will test these hypotheses in the following specific aims: AIM 1: To determine if Pdgfra+ fibroblasts give rise to myofibroblasts after bleomycin lung injury. AIM 2: To test the hypothesis that Pdgfra+ fibroblasts are a major source of the pro-fibrotic mediators HA and Has2 in the development of bleomycin-induced pulmonary fibrosis. AIM 3: To test the hypothesis that primary human PDGFRa+ fibroblasts support the survival, proliferation, and differentiation of human AEC2s, thereby serving as components of the AEC2 niche.